1. Field of the Invention
The present invention relates to sampling pumps of the type commonly used to intermittently withdraw sample hydrocarbon fluids from a pipeline and input those fluids to a sample vessel for subsequent analysis, and more particularly relates to improved techniques for mounting the sample pump and related equipment directly to the pipeline.
2. Description of the Background
Sampling pumps have been used for years to intermittently withdraw fluid samples from a hydrocarbon pipeline and input those samples to a suitable container for subsequent analysis. The price charged for hydrocarbons being transmitted through the pipeline is typically determined as a function of (a) the volume of the actual fluid transferred through the pipeline, and (b) the BTU content of sample fluid intermittently withdrawn from the pipeline. The sample fluid vessel is periodically transported to a laboratory for fluid analysis by any suitable instrument, such as a gas chromatograph. The accuracy of the sampling technique, i.e., the caloric or BTU value of the fluid sample in the vessel compared to the average BTU value of all the fluid transmitted through that pipeline, thus has a significant affect on the price paid for the transmitted fluid. Accordingly numerous attempts have been made and will continue to increase the reliability and accuracy of this sampling technique, while at the same time lowering the overall sampling costs
One version of a sampling pump is disclosed in U.S. Pat. No. 4,403,518. U.S. Pat. No. 4,470,773 discloses a similar pump with a feature for breaking the vacuum in the pump which could otherwise be created by the retraction of the collection probe from the resilient plug. The prime mover for reciprocating the sampling pump piston is typically a diaphragm, which in turn is powered by a pressurized air source, as disclosed in U.S. Pat. Nos. 4,440,032 and 4,525,127. U.S. Pat. No. 4,557,157 discloses a sampling pump which also utilizes this type of prime mover, and further discloses a pressure balanced check valve external of the pump body. This check valve utilizes line pressure as a reference, and enables the pump to perform its desired function of transmitting a preselected fluid volume per stroke regardless of line pressure.
Most sampling pumps are primarily intended to pump
either a liquid or a gas, but not both. A representative liquid sampling pump is disclosed in U.S. Pat. No. 4,172,670, while a gas sampling pump is described in U.S. Pat. No. 4,531,895. A combined pump and sample vessel is disclosed in U.S. Pat. No. 4,628,750. Operators of sampling pumps frequently prefer a "hot loop" between the pipeline and the pump. The hot loop flows a slight amount of fluid continuously from the pipeline to the generally vicinity of the pump, so that the pump is sampling fluid reasonably respresentative of the fluid flowing through the pipeline at each sampling period. If the hot loop is not provided, fluid is drawn with each pump stroke into the flow line between the pipeline and the sampling pump, but the pump is then transmitting fluid into the sample container from previous pump strokes, which inherently reduces the accuracy of the sampling technique. In the hot loop system, fluid which is not sampled may be returned to the pipeline.
It is conventional to house the sampling pump, the pump actuator, a regulator, and an electronic sampling controller within a single protective housing, and then connect ports extending from the housing to the pipeline by flexible tubing. The protective housing is thus generally mounted separate from the pipeline. Fluid sampling pump manufacturers have recently promoted systems which mount the pump directly to the pipeline, thereby reducing installation cost. Previous direct mounting systems for sampling pump have significant disadvantages, however, which have reduced their commercial acceptance. While a new pump and manifold assembly may be manufactured and sent to the field to allow direct mounting of the pump to the pipeline via the manifold, such a system does not practically benefit an operator with an existing conventional sampling pump housed within a protective enclosure with related equipment, since the existing pump has to be disconnected from the related equipment and effectively removed from the enclosure to install the new manifold on the pump, then the assembly has to be reconnected and remounted within the enclosure so that the pump within the enclosure is now directly mounted to the pipeline. The cost and time associated with such disconnection, disassembly, and reconnection operations typically does not justify the benefit obtained by direct mounting of the pump, especially since some provisions previously had to made to support the pump and enclosure separate from the pipeline.
Another disadvantage of existing systems directly connecting a pump to a pipeline is that the manifold is rotatably fixed to the pump prior to shipping the assembly to the field for installation, and the rotatable position of the installed manifold is uncontrollably determined by the tapered threads which connect the manifold to the pipeline nipple. Tapered or NPT threads should be "made up" within a certain torque range and neither under torqued (which may result in fluid leakage) or over torqued, which could cause fracture of the tapered threads in the manifold or the mating threads on the nipple. Since the pump is rotatably fixed to the manifold, the rotational position of the pump is determined by the tapered threads, and the position of manifold cannot be effectively predetermined. The operator in the field, however, almost always prefers that the face of the pump and/or the face of the enclosure housing the pump and related equipment be at a certain position, so that the controller within the enclosure can be easily adjusted and the components within the enclosure checked. Accordingly, installation personnel tend to over-tighten the tapered threads between the manifold and the pipeline nipple to force the enclosure face to be at the desired orientation. This over-tightening can cause failure which may not occur at the time of the installation, and accordingly the installed system may first begin to leak or rupture months after installation due to over tightening of the manifold threads to the pipeline nipple.
The disadvantages of the prior art are overcome by the present invention, and an improved sampling pump and direct mount system for a sampling pump is hereinafter disclosed. The pump of the present invention is suitable for reliably withdrawing various sample fluids from a pipeline at various line pressures and inputting those fluids to a desired sample vessel. The direct mount system enhances the versatility of the sampling procedure, reduces installation costs, minimizes maintainance, and may be easily adapted to existing systems.